Imaging lens

ABSTRACT

An imaging lens includes a first lens having negative refractive power; an aperture stop; a second lens; a third lens; a fourth lens having negative refractive power; a fifth lens; and a sixth lens, arranged in this order from an object side to an image plane side. The fifth lens is formed in a shape so that a surface thereof on the object side is convex at a paraxial region thereof. The sixth lens is formed in a meniscus shape at a paraxial region thereof. The first lens and the sixth lens have specific Abbe&#39;s numbers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of a prior application Ser. No.16/165,182, filed on Oct. 19, 2018, pending.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an imaging lens for forming an image ofan object on an imaging element such as a CCD sensor and a CMOS sensor.In particular, the present invention relates to an imaging lens suitablefor mounting in a relatively small camera such as a camera to be builtin an onboard camera, a smartphone, a cellular phone, a digital camera,a digital video camera, a network camera, a TV conference camera, afiber scope, and a capsule endoscope.

In these years, some vehicles are equipped with a plurality of camerasfor improving safety and/or convenience. For example, as a camera tosupport safely backing a vehicle, there is a backup camera. For example,in case of a vehicle equipped with a backup camera, a view behind thevehicle is displayed on a monitor upon backing the vehicle. Since adriver can visually see an object even if the object is invisible due toshadow behind the vehicle, the driver can safely back the vehiclewithout hitting an obstacle. Such a camera for mounting in a vehicle,so-called “onboard camera”, is expected to be continuously in moredemand.

Such onboard cameras may be mounted in relatively small spaces, such asa back door, a front grille, a side mirror, and interior space thereof.For this reason, an imaging lens for mounting in an onboard camera isstrongly required to have a compact size, in addition to attaining ahigh resolution suitable for a higher pixel count of an imaging elementand a wide angle of view to achieve a wider range of an image. However,when the downsizing of the imaging lens is attempted, each lens hasstrong refractive power, and it is often difficult to satisfactorilycorrect aberrations. Upon actual designing of an imaging lens, it is akey to achieve those multiple requirements such as satisfactorycorrection of aberrations, a wider angle of view, and downsizing in abalanced manner.

On the other hand, in place of cellular phones that are intended mainlyfor making phone calls, so-called “smartphones, i.e., multifunctionalcellular phones which can run various application software as well as avoice call function, have been more widely used. The product lineup ofsmartphones is very wide including beginner models to high-end models,and is often categorized by performance of hardware, optical performanceof a camera, etc. Among those high-end models, there is a model that isintended to have new added value by being equipped with two imaginglenses. For example, in case of a model having an imaging lens with awide angle of view as well as a conventional imaging lens with a typicalangle of view, images from those two imaging lenses are synthesized byprocessing software so as to achieve smooth zooming in and zooming out.In case of those imaging lenses for such a purpose, similarly to theimaging lenses of the onboard camera, it is required to have a smallersize of the imaging lens, as well as satisfactory correction ofaberrations and a wider angle of view.

For example, as the conventional imaging lens having a wide angle ofview, an imaging lens described in Patent Reference has been known.

PATENT REFERENCE

Patent Reference: Japanese Patent Application Publication No.2017-037119

The conventional imaging lens described in Patent Reference includes afirst lens, a second lens, a third lens, a stop, a fourth lens, a fifthlens, and a sixth lens arranged in the order from an object side. Thefirst lens has a concave surface on an image plane side and has negativerefractive power. The second lens is a meniscus lens directing a concavesurface thereof to the object side. The third lens has a biconvex shapenear the optical axis and has positive refractive power. The fourth lenshas negative refractive power, and the fifth lens has positiverefractive power. The sixth lens has at least one aspheric surface. Ingeneral, when an imaging lens is designed to have a wide angle, thefirst lens tends to have a large outer shape. In this point, accordingto the conventional imaging lens of Patent Reference, while having awide angle, the enlargement of the first lens is restricted and thefield curvature is satisfactorily corrected.

According to the conventional imaging lens disclosed in PatentReference, although the number of the lenses that composes theconventional imaging lens is as small as six, the angle of view is wideand aberrations are relatively satisfactorily corrected. However, sincethe total length of the lens system is increased, the imaging lens doesnot satisfy the requirements of downsizing in these days. Further, it isdifficult to satisfy both a small size of the imaging lens andsatisfactory correction of aberrations. Here, such a problem is notspecific to the imaging lens to be mounted in onboard cameras andsmartphones. Rather, it is a common problem for an imaging lens to bemounted in a relatively small camera such as cellular phones, digitalcameras, digital video cameras, network cameras, TV conference cameras,fiber scopes, and capsule endoscopes.

In view of the above-described problems in the conventional techniques,an object of the present invention is to provide an imaging lens thatcan attain a wider angle of view and satisfactory correction ofaberrations, while achieving a small size.

Further objects and advantages of the present invention will be apparentfrom the following description of the invention.

SUMMARY OF THE INVENTION

In order to attain the objects described above, according to a firstaspect of the present invention, an imaging lens includes a first lens,a second lens, a third lens, a fourth lens, a fifth lens, and a sixthlens in the order from an object side to an image plane side. The firstlens has negative refractive power. The second lens has positiverefractive power. The third lens has positive refractive power. Thefourth lens has negative refractive power. The sixth lens has negativerefractive power. The fourth lens and the fifth lens are disposed so asto face each other. An image plane-side surface of the fourth lens isformed in a concave shape at the peripheral portion thereof. Anobject-side surface of the fifth lens is formed in a concave shape atthe peripheral portion thereof.

In attaining a wider angle of view of the imaging lens, correction of afield curvature and a distortion is especially important. According tothe imaging lens of the present invention, the refractive power of thefour lenses is arranged in a well-balanced manner such asnegative-positive-positive-negative in the order from the object side.Accordingly, the aberrations generated in the first lens are suitablycorrected by two lenses having positive refractive power and the fourthlens having negative refractive power. In addition, according to theimaging lens of the present invention, the fourth lens is formed in ashape such that an image plane side surface thereof is concave at theperipheral portion thereof. Moreover, the fifth lens, arranged facingthe fourth lens, is formed in a shape such that an object side surfacethereof is concave at the peripheral portion thereof. With such shapesof the fourth lens and the fifth lens, it is achievable to downsize theimaging lens and to satisfactorily correct the field curvature and thedistortion.

When a composite focal length of the first lens, the second lens, thethird lens and the fourth lens is F1, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (1):

0<F1  (1)

When the imaging lens of the present invention satisfies the conditionalexpression (1), it is achievable to downsize the imaging lens and tosatisfactorily correct an astigmatism, the distortion, and the chromaticaberration. When the composite focal length F1 is outside the range ofthe conditional expression (1) and is negative, the composite refractivepower of the fifth lens and the sixth lens is positive and therefractive power is strong. Therefore, it is difficult to downsize theimaging lens. In addition, the distortion increases in the negativedirection and the astigmatic difference increases. An axial chromaticaberration is excessively corrected (a focal position at a shortwavelength moves to the image plane side relative to a focal position ata reference wavelength), and a chromatic aberration of magnification isinsufficiently corrected (an image-forming point at a short wavelengthis close to the optical axis relative to an image-forming point at areference wavelength). For this reason, it is difficult to obtainsatisfactory image-forming performance.

When the composite focal length of the fifth lens and the sixth lens isF2, the imaging lens having the above-described configuration preferablysatisfies the following conditional expression (2):

F2<0  (2)

When the imaging lens satisfies the conditional expression (2), it isachievable to satisfactorily correct the astigmatism and the distortion,while downsizing the imaging lens. When the value of the composite focallength F2 is outside the range of the conditional expression (2), it isadvantageous for correction of distortion, but it is difficult todownsize the imaging lens. In addition, a sagittal image surface of theastigmatism curves to the object side, the astigmatic differenceincreases, and it is difficult to correct the field curvature.Therefore, it is difficult to obtain satisfactory image-formingperformance.

According to the imaging lens having the above-described configuration,it is preferred to dispose an aperture stop between the first lens andthe second lens. Here, in this specification, “between the first lensand the second lens” means between a tangential plane of a vertex of theobject-side surface of the first lens and a tangential plane of a vertexof the image plane-side surface of the second lens. Disposing theaperture stop in such a position, it is achievable to suitably restrainan incident angle of a light beam emitted from the imaging lens to theimage plane of the imaging element within the range of chief ray angle(CRA) and to satisfactorily correct aberrations.

When the whole lens system has the focal length f and a distance on theoptical axis between the first lens and the second lens is D12, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (3):

0.05<D12/f<0.8  (3)

When the imaging lens satisfies the conditional expression (3), it isachievable to satisfactorily correct the chromatic aberration, the fieldcurvature, and the astigmatism. When the value exceeds the upper limitof “0.8”, although it is advantageous for correction of the chromaticaberration of magnification, the sagittal image surface of theastigmatism curves to the image plane side and the astigmatic differenceincreases. In addition, the image-forming surface curves to the imageplane side and the field curvature is excessively corrected. On theother hand, when the value is below the lower limit of “0.05”, thechromatic aberration of magnification is excessively corrected (theimage-forming point at a short wavelength moves in a direction to beaway from the optical axis relative to the image-forming point at areference wavelength), and the sagittal image surface of the astigmatismcurves to the object side and the astigmatic difference increases. Inaddition, the image-forming surface curves to the object side, and thefield curvature is insufficiently corrected. Therefore, in either case,it is difficult to obtain satisfactory image-forming performance.

When the whole lens system has the focal length f and a distance on theoptical axis between the second lens and the third lens is D23, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (4):

0.001<D23/f<0.3  (4)

When the value satisfies the conditional expression (4), it isachievable to satisfactorily correct the chromatic aberration, the fieldcurvature, and the astigmatism. When the value exceeds the upper limitof “0.3”, the chromatic aberration of magnification is excessivelycorrected and the sagittal image surface of the astigmatism curves tothe image plane side and the astigmatic difference increases. Inaddition, the field curvature is excessively corrected, so that it isdifficult to obtain satisfactory image-forming performance. On the otherhand, when the value is below the lower limit of “0.001”, the axialchromatic aberration is excessively corrected and the astigmaticdifference increases. Therefore, also in this case, it is difficult toobtain satisfactory image-forming performance.

When the distance on the optical axis between the first lens and thesecond lens is D12 and the distance on the optical axis between thesecond lens and the third lens is D23, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (5):

2<D12/D23<30  (5)

When the imaging lens satisfies the conditional expression (5), it isachievable to satisfactorily correct the distortion, the fieldcurvature, and the astigmatism, while downsizing the imaging lens. Whenthe value exceeds the upper limit of “30”, it is difficult to downsizethe imaging lens and the distortion increases in the negative direction.In addition, the sagittal image surface of the astigmatism curves to theimage plane side, and the astigmatic difference increases. The fieldcurvature is excessively corrected. On the other hand, when the value isbelow the lower limit of “2”, the sagittal image surface of theastigmatism curves to the object side, and the astigmatic differenceincreases. Therefore, in either case, it is difficult to obtainsatisfactory image-forming performance.

When the second lens has a focal length f2 and the third lens has afocal length f3, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(6):

0.5<f2/f3<3  (6)

When the imaging lens satisfies the conditional expression (6), it isachievable to satisfactorily correct the astigmatism and the fieldcurvature, while downsizing the imaging lens. When the value exceeds theupper limit of “3”, it is difficult to downsize the imaging lens. Inaddition, the sagittal image surface of the astigmatism curves to theimage plane side, so that the astigmatic difference increases and thefield curvature is excessively corrected. Therefore, it is difficult toobtain satisfactory image-forming performance. On the other hand, whenthe value is below the lower limit of “0.5”, it is advantageous fordownsizing of the imaging lens. However, the astigmatic differenceincreases and the field curvature is insufficiently corrected.Therefore, it is difficult to obtain satisfactory image-formingperformance.

When the whole lens system has a focal length f and a composite focallength of the second lens and the third lens is f23, the imaging lenshaving the above-described configuration preferably satisfies thefollowing conditional expression (7):

0.2<f23/f<1  (7)

When the imaging lens satisfies the conditional expression (7), it isachievable to satisfactorily correct the chromatic aberration, theastigmatism, the distortion, and the field curvature, while suitablyrestraining the incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. When the value exceedsthe upper limit of “1”, although it is advantageous for correction ofthe chromatic aberration, the distortion increases in the negativedirection. In addition, the sagittal image surface of astigmatism curvesto the object side, the astigmatic difference increases. On the otherhand, when the value is below the lower limit of “0.2”, the axialchromatic aberration and the chromatic aberration of magnification areboth excessively corrected. In addition, the sagittal image surface ofthe astigmatism curves to the image plane side, so that the astigmaticdifference increases and the field curvature is excessively corrected.Therefore, in either case, it is difficult to obtain satisfactoryimage-forming performance.

In order to correct the aberrations more satisfactorily, the imaginglens having the above-described configuration preferably satisfies thefollowing conditional expression (7A):

0.2<f23/f<0.8  (7A)

When the first lens has Abbe's number ν1, the second lens has Abbe'snumber ν2, and the third lens has Abbe's number ν3, the imaging lenshaving the above-described configuration preferably satisfies thefollowing conditional expressions (8) through (10):

40<ν1<75  (8)

40<ν2<75  (9)

40<ν3<75  (10)

When the imaging lens satisfies the conditional expressions (8) through(10), it is achievable to satisfactorily correct the chromaticaberration. When the three lenses from the object side are formed oflow-dispersion materials, it is achievable to suitably restraingeneration of the chromatic aberration in the imaging lens. When thevalue exceeds the upper limit of “75”, although it is advantageous forcorrection of the axial chromatic aberration, the chromatic aberrationof magnification excessively corrected. Therefore, it is difficult toobtain satisfactory image-forming performance. On the other hand, whenthe value is below the lower limit of “40”, although it is advantageousfor correction of the chromatic aberration of magnification, the axialchromatic aberration is excessively corrected. Therefore, also in thiscase, it is difficult to obtain satisfactory image-forming performance.

When the whole lens system has a focal length f and the fourth lens hasa focal length f4, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(11):

−4<f4/f<−0.4  (11)

When the imaging lens satisfies the conditional expression (11), it isachievable to satisfactorily correct the chromatic aberration, theastigmatism, and the field curvature, while suitably restraining theincident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA. When the imaging lens satisfies theupper limit of “−0.4”, it is difficult to restrain the incident angleemitted from the imaging lens to the image plane within the range ofCRA. In addition, the axial chromatic aberration and the chromaticaberration of magnification are both excessively corrected and thesagittal image surface of the astigmatism curves to the image plane sideso that the astigmatic difference increases. On the other hand, when thevalue is below the lower limit of “−4”, while it is easy to restrain theincident angle of the light beam to the image plane within the range ofCRA, the sagittal image surface of the astigmatism curves to the objectside and the astigmatic difference increases. Therefore, in either case,it is difficult to obtain satisfactory image-forming performance.

According to the imaging lens having the above-described configuration,the fifth lens is preferably formed in an aspheric shape such that boththe object-side surface and the image plane-side surface thereof have aninflection point. With such a shape of the fifth lens, it is achievableto suitably restrain the incident angle of a light beam emitted from theimaging lens to the image plane within the range of CRA.

According to the imaging lens having the above-described configuration,the sixth lens is preferably formed in an aspheric shape such that theimage plane-side surface thereof has an inflection point. With such ashape of the sixth lens, it is achievable to more suitably restrain theincident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA.

When the whole lens system has the focal length f and a distance on theoptical axis between the fourth lens and the fifth lens is D45, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (12):

0.15<D45/f<0.4  (12)

When the imaging lens satisfies the conditional expression (12), it isachievable to satisfactorily correct the chromatic aberration and theastigmatism, while downsizing the imaging lens. When the imaging lenssatisfies the conditional expression (12), it is achievable to suitablyrestrain the incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. When the value exceedsthe upper limit of “0.4”, while it is easy to restrain the incidentangle of the light beam to the image plane within the range of CRA, theastigmatic difference increases. In addition, the chromatic aberrationof magnification is excessively corrected, and it is it is difficult toobtain satisfactory image-forming performance. On the other hand, whenthe value is below the lower limit of “0.15”, while it is advantageousto downsize the imaging lens, it is difficult to restrain the incidentangle of the light beam within the range of CRA. In addition, theastigmatic difference and the axial chromatic aberration increase.Therefore, it is difficult to obtain satisfactory image-formingperformance.

When the distance on the optical axis between the fourth lens and thefifth lens is D45 and the distance on the optical axis from theobject-side surface of the first lens to the image plane-side surface ofthe sixth lens is L16, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(13):

0.05<D45/L16<0.25  (13)

When the imaging lens satisfies the conditional expression (13), it isachievable to satisfactorily correct the chromatic aberration and theastigmatism, while downsizing the imaging lens. When the imaging lenssatisfies the conditional expression (13), it is achievable to suitablyrestrain the incident angle of a light beam emitted from the imaginglens to the image plane within the range of CRA. When the value exceedsthe upper limit of “0.25”, while it is easy to restrain the incidentangle of the light beam to the image plane within the range of CRA, theastigmatic difference increases. In addition, the chromatic aberrationof magnification is excessively corrected, and it is it is difficult toobtain satisfactory image-forming performance. On the other hand, whenthe value is below the lower limit of “0.05”, while it is advantageousto downsize the imaging lens, it is difficult to restrain the incidentangle of the light beam within the range of CRA. In addition, theastigmatic difference and the axial chromatic aberration increase.Therefore, it is difficult to obtain satisfactory image-formingperformance.

When the image plane side surface of the fourth lens has an effectivediameter Φ4B and the object side surface of the fifth lens has aneffective diameter Φ5A, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(14):

1<Φ5A/Φ4B<2  (14)

When the imaging lens satisfies the conditional expression (14), it isachievable to suitably restrain the incident angle of a light beamemitted from the imaging lens to the image plane within the range ofCRA, while achieving small size and a wide angle of view of the imaginglens. When the value exceeds the upper limit of “2”, a differencebetween the effective diameter Φ4B and the effective diameter Φ5A islarge, and the incident angle of the light beam emitted from the imaginglens to the image plane is large. Therefore, it is difficult to restrainthe incident angle within the range of CRA. On the other hand, when thevalue is below the lower limit of “1”, while it is easy to restrain theincident angle within the range of CRA, it is difficult to achieve smallsize and a wide angle of view of the imaging lens.

When the whole lens system has a focal length f and the sixth lens has afocal length f6, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(15):

−5<f6/f<−0.5  (15)

When the imaging lens satisfies the conditional expression (15), it isachievable to satisfactorily correct the astigmatism and the distortion,while downsizing the imaging lens. When the imaging lens satisfies theconditional expression (15), it is achievable to suitably restrain theincident angle of a light beam emitted from the imaging lens to theimage plane within the range of CRA. When the value exceeds the upperlimit of “−0.5”, although it is advantageous for downsizing of theimaging lens, the astigmatic difference increases. Therefore, it isdifficult to obtain satisfactory image-forming performance. On the otherhand, when the value is below the lower limit of “−5”, it is easy torestrain the incident angle of the light beam to the image plane withinthe range of CRA. However, the sagittal image surface of the astigmatismcurves to the object side, so that the astigmatic difference increases.Therefore, it is difficult to obtain satisfactory image-formingperformance.

When the sixth lens has Abbe's number ν6, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (16):

10<ν6<40  (16)

When the imaging lens satisfies the conditional expression (16), it isachievable to satisfactorily correct the chromatic aberration. When thevalue exceeds the upper limit of “40”, although it is easy to correctthe axial chromatic aberration, the chromatic aberration ofmagnification is excessively corrected. Therefore, it is difficult toobtain satisfactory image-forming performance. On the other hand, whenthe value is below the lower limit of “10”, the axial chromaticaberration is excessively corrected and the chromatic aberration ofmagnification increases. For this reason, it is difficult to obtainsatisfactory image-forming performance.

When the distance on the optical axis between the first lens and thesecond lens is D12 and the distance on the optical axis between thefifth lens and the sixth lens is D56, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (17):

D56<D12  (17)

When the imaging lens satisfies the conditional expression (17), it isachievable to satisfactorily correct the astigmatism, while downsizingthe imaging lens. When the value is outside the range of the conditionalexpression (17), it is advantageous for downsizing of the imaging lens.However, the sagittal image surface of the astigmatism curves to theobject side, and the astigmatic difference increases and the fieldcurvature is insufficiently corrected. Therefore, it is difficult toobtain satisfactory image-forming performance.

When the whole lens system has the focal length f and the distance onthe optical axis between the fifth lens and the sixth lens is D56, theimaging lens having the above-described configuration preferablysatisfies the following conditional expression (18):

0.03<D56/f<0.3  (18)

When the imaging lens satisfies the conditional expression (18), it isachievable to satisfactorily correct the astigmatism and the chromaticaberration of magnification. When the value exceeds the upper limit of“0.3”, although it is easy to correct the astigmatism, the chromaticaberration of magnification increases. On the other hand, when the valueis below the lower limit of “0.03”, although it is advantageous tocorrect the chromatic aberration of magnification, the astigmatismincreases at the peripheral portion thereof. Therefore, in either case,it is difficult to obtain satisfactory image-forming performance.

When the thickness of the fifth lens on the optical axis is T5 and thethickness of the sixth lens on the optical axis is T6, the imaging lenshaving the above-described configuration preferably satisfies thefollowing conditional expression (19):

0.5<T5/T6<3.5  (19)

When the imaging lens satisfies the conditional expression (19), it isachievable to satisfactorily correct the field curvature and theastigmatism. When the value exceeds the upper limit of “3.5”, the fieldcurvature is excessively corrected. In addition, the sagittal imagesurface of the astigmatism tilts to the image plane side and theastigmatic difference increases. On the other hand, when the value isbelow the lower limit of “0.5”, the field curvature is insufficientlycorrected and the astigmatism increases. Therefore, in either case, itis difficult to obtain satisfactory image-forming performance.

When the object-side surface of the first lens has an effective diameterΦ1A and the image plane-side surface of the sixth lens has an effectivediameter Φ6B, the imaging lens having the above-described configurationpreferably satisfies the following conditional expression (20):

Φ1A<Φ6B  (20)

The object-side surface of the first lens disposed closest to the objectside may be exposed to the most severe environment, such as exposure toliquid, e.g., water. In case of a conventional wide-angle imaging lens,typically, the first lens is large. Therefore, when such conventionalimaging lens is mounted in a vehicle for an onboard camera, the sizecould be an issue. Moreover, on an object-side surface of the firstlens, an optical thin film may be frequently formed so as to attainenvironmental resistance. When the imaging lens satisfies theconditional expression (20), it is achievable to reduce an exposed areaof the first lens, which contacts with surrounding environment. With thesmall diameter of the first lens, it is achievable to reduce the cost ofthe optical thin film, which in turn reduces the manufacturing cost ofthe imaging lens.

When the distance on the optical axis from the object-side surface ofthe first lens to the image plane is La and the maximum image height ofthe image plane is Hmax, the imaging lens having the above-describedconfiguration preferably satisfies the following conditional expression(21):

0.4<La/H max<1.8  (21)

In case of an imaging lens to be mounted in a thin smartphone or thelike, it is necessary to hold the imaging lens within a limited space.Therefore, the total length of the imaging lens is strictly limited.Furthermore, as the angle of view of the imaging lens is wider, it isimportant not merely to have a small size but also to make the ratio ofthe total length of the imaging lens to the size of the image plane assmall as possible, i.e., how to achieve low profile is important. Whenthe imaging lens satisfies the conditional expression (21), it ispossible to attain low profile of the imaging lens. Here, between theimaging lens and the image plane, typically, there is often disposed aninsert such as an infrared cut-off filter and cover glass. In thisspecification, for the distance on the optical axis of those inserts, anair conversion length is employed.

According to the present invention, the respective lenses from the firstlens to the sixth lens are preferably disposed with certain airintervals. When the respective lenses are disposed at certain airintervals, the imaging lens of the present invention can have a lensconfiguration that does not contain any cemented lens. In such lensconfiguration like this, since it is easy to form all of the six lensesthat compose the imaging lens from plastic materials, it is achievableto suitably restrain the manufacturing cost of the imaging lens.

Recently, an imaging element with high pixel count is more frequentlycombined with an imaging lens for a purpose of improving performance ofa camera. In case of such an imaging element with a high pixel count, alight-receiving area of each pixel decreases, so that an image tends tobe dark. As a method of correcting the darkness of the image, there is amethod of improving light-receiving sensitivity of the imaging elementby using an electrical circuit. However, when the light-receivingsensitivity increases, a noise component, which does not directlycontribute to formation of an image, is also amplified. Therefore, it isnecessary to have another circuit to reduce the noise. Accordingly, inorder to obtain fully bright image without such electrical circuit, whenthe whole lens system has the focal length f and the imaging lens has adiameter of entrance pupil Dep, the imaging lens having theabove-described configuration preferably satisfies the followingconditional expression (22):

f/Dep<2.5  (22)

According to the present invention, when the imaging lens has an angleof view 2ω, the imaging lens preferably satisfies 100°≤2ω. When theimaging lens satisfies this conditional expression, it is possible toattain a wide angle of the imaging lens and it is achievable to suitablyattain both downsizing and the wide angle of the imaging lens.

According to the present invention, as described above, the shapes ofthe lenses are specified using positive/negative signs of the curvatureradii thereof. Whether the curvature radius of the lens is positive ornegative is determined based on general definition. More specifically,taking a traveling direction of light as positive, if a center of acurvature radius is on the image plane side when viewed from a lenssurface, the curvature radius is positive. If a center of a curvatureradius is on the object side, the curvature radius is negative.Therefore, “an object-side surface having a positive curvature radius”means the object-side surface is a convex surface. “An object-sidesurface having a negative curvature radius” means the object sidesurface is a concave surface. In addition, “an image plane-side surfacehaving a positive curvature radius” means the image plane-side surfaceis a concave surface. “An image plane-side surface having a negativecurvature radius” means the image plane-side surface is a convexsurface. Here, a curvature radius used herein refers to a paraxialcurvature radius, and may not fit to general shapes of the lenses intheir sectional views all the time.

According to the imaging lens of the present invention, it is achievableto provide an imaging lens having a wide angle of view, which isespecially suitable for mounting in a small-sized camera, while havinghigh resolution with satisfactory correction of aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 1 of the present invention;

FIG. 2 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 1;

FIG. 3 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 1;

FIG. 4 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 2 of the present invention;

FIG. 5 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 4;

FIG. 6 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 4;

FIG. 7 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 3 of the present invention;

FIG. 8 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 7;

FIG. 9 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 7;

FIG. 10 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 4 of the present invention;

FIG. 11 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 10;

FIG. 12 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 10;

FIG. 13 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 5 of the present invention;

FIG. 14 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 13;

FIG. 15 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 13;

FIG. 16 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 6 of the present invention;

FIG. 17 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 16;

FIG. 18 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 16;

FIG. 19 is a sectional view showing a schematic configuration of animaging lens in Numerical Data Example 7 of the present invention;

FIG. 20 is an aberration diagram showing a lateral aberration of theimaging lens of FIG. 19; and

FIG. 21 is an aberration diagram showing a spherical aberration,astigmatism, and a distortion of the imaging lens of FIG. 19.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, referring to the accompanying drawings, an embodiment of thepresent invention will be fully described.

FIGS. 1, 4, 7, 10, 13, 16, and 19 are schematic sectional views of theimaging lenses in Numerical Data Examples 1 to 7 according to theembodiment, respectively. Since the imaging lenses in those NumericalData Examples have the same basic configuration, the lens configurationof the embodiment will be described with reference to the illustrativesectional view of Numerical Data Example 1.

The imaging lens of the embodiment includes a first lens L1 havingnegative refractive power; a second lens L2 having positive refractivepower; a third lens L3 having positive refractive power; a fourth lensL4 having negative refractive power; a fifth lens L5, and a sixth lensL6 having negative refractive power, arranged in the order from anobject side. The fourth lens L4 and the fifth lens L5 are disposed toface each other. Between the first lens L1 and the second lens L2, thereis disposed an aperture stop ST. In addition, between the sixth lens L6and an image plane IM of an imaging element, there is provided a filter10. The filter 10 is omissible.

The first lens L1 is formed in a shape such that a curvature radius r1of a surface thereof on the object-side and a curvature radius r2 of asurface thereof on the image plane side are both positive, so as to havea shape of a meniscus lens directing a convex surface thereof to theobject side near an optical axis. The shape of the first lens L1 may notbe limited to the one in Numerical Data Example 1. The first lens L1 canbe formed in a shape such that the curvature radius r1 is negative andthe curvature radius r2 is positive, so as to have a shape of abiconcave lens near the optical axis. The first lens L1 can be formed inany shape as long as the curvature radius r2 of the image plane sidesurface thereof is positive.

The second lens L2 is formed in a shape such that a curvature radius r4of a surface thereof on the object-side is positive and a curvatureradius r5 of a surface thereof on the image plane side is negative, soas to have a shape of a biconvex lens near the optical axis. The shapeof the second lens L2 may not be limited to the one in Numerical DataExample 1. The second lens L2 can be formed in a shape of a meniscuslens directing a convex surface thereof to the object side near theoptical axis, or can be formed in a shape of a meniscus lens directing aconcave surface thereof to the object side near the optical axis.

The third lens L3 is formed in a shape such that a curvature radius r6of a surface thereof on the object-side is positive and a curvatureradius r7 of a surface thereof on the image plane side is negative, soas to have a shape of a biconvex lens near the optical axis. The shapeof the third lens L3 may not be limited to the one in Numerical DataExample 1. For example, the third lens L3 can be formed in a shape suchthat the curvature radius r6 and the curvature radius r7 are bothnegative, so as to have a shape of a meniscus lens directing a concavesurface thereof to the object side near the optical axis. The third lensL3 of Numerical Data Example 6 is an example of a shape of a meniscuslens directing a concave surface thereof to the object side near theoptical axis.

The fourth lens L4 is formed in a shape such that a surface thereof onthe image plane side is concave at the peripheral portion thereof.According to the imaging lens of Numerical Data Example 1, the fourthlens L4 is formed in a shape such that a curvature radius r8 of asurface thereof on the object-side and a curvature radius r9 of asurface thereof on the image plane side are both positive, so as to havea shape of a meniscus lens directing a convex surface thereof to theobject side near the optical axis. Here, the shape of the fourth lens L4is not limited to the one in Numerical Data Example 1. The fourth lensesL4 of Numerical Data Examples 2 through 5 are examples of shapes of abiconcave lens near the optical axis, and the fourth lens L4 ofNumerical Data Example 7 is an example of a shape of a meniscus lensdirecting a concave surface thereof to the object side near the opticalaxis.

The fifth lens L5 is formed in an aspheric shape such that the bothobject-side surface and the image plane-side surface thereof have aninflection point and the object-side surface thereof is concave at theperipheral portion thereof. According to the imaging lens of NumericalData Example 1, the fifth lens L5 is formed in a shape such that acurvature radius r10 of a surface thereof on the object-side and acurvature radius r11 of a surface thereof on the image plane side areboth positive, so as to have a shape of a meniscus lens directing aconvex surface thereof to the object side near the optical axis. Here,the shape of the fifth lens L5 may not be limited to the one inNumerical Data Example 1.

The sixth lens L6 is formed in an aspheric shape such that a surfacethereof on the image plane side has an inflection point. According tothe imaging lens of Numerical Data Example 1, the sixth lens L6 isformed in a shape such that a curvature radius r12 of a surface thereofon the object-side and a curvature radius r13 of a surface thereof onthe image plane side are both positive, so as to have a shape of ameniscus lens directing a convex surface thereof to the object side nearthe optical axis. Here, the shape of the sixth lens L6 may not belimited to the one in Numerical Data Example 1. The sixth lens L6 ofNumerical Data Example 4 is an example of a shape of a biconcave lensnear the optical axis. On the other hand, the sixth lens L6 of NumericalData Example 7 is an example of a shape of a meniscus lens directing aconcave surface thereof to the object side near the optical axis.

According to the embodiment, the imaging lens satisfied the followingconditional expressions (1) through (22):

0<F1  (1)

F2<0  (2)

0.05<D12/f<0.8  (3)

0.001<D23/f<0.3  (4)

2<D12/D23<30  (5)

0.5<f2/f3<3  (6)

0.2<f23/f<1  (7)

0.2<f23/f<0.8  (7A)

40<ν1<75  (8)

40<ν2<75  (9)

40<ν3<75  (10)

−4<f4/f<−0.4  (11)

0.15<D45/f<0.4  (12)

0.05<D45/L16<0.25  (13)

1<Φ5A/Φ4B<2  (14)

−5<f6/f<−0.5  (15)

10<ν6<40  (16)

D56<D12  (17)

0.03<D56/f<0.3  (18)

0.5<T5/T6<3.5  (19)

Φ1A<Φ6B  (20)

0.4<La/H max<1.8  (21)

f/Dep<2.5  (22)

In the above conditional expressions:

f: Focal length of the whole lens systemf2: Focal length of the second lens L2f3: Focal length of the third lens L3f4: Focal length of the fourth lens L4f6: Focal length of the sixth lens L6f23: Composite focal length of the second lens L2 and the third lens L3F1: Composite focal length of the first lens L1 through the fourth lensL4F2: Composite focal length of the fifth lens L5 and the sixth lens L6T5: Thickness of the fifth lens L5 on an optical axisT6: Thickness of the sixth lens L6 on an optical axisΦ1A: Effective diameter of an object-side surface of the first lens L1Φ4B: Effective diameter of an image plane-side surface of the fourthlens L4Φ5A: Effective diameter of an object-side surface of the fifth lens L5Φ6B: Effective diameter of an image plane-side surface of the sixth lensL6D12: Distance along the optical axis X between the first lens L1 and thesecond lens L2D23: Distance along the optical axis X between the second lens L2 andthe third lens L3D45: Distance along the optical axis X between the fourth lens L4 andthe fifth lens L5D56: Distance along the optical axis X between the fifth lens L5 and thesixth lens L6L16: Distance along the optical axis X between the object-side surfaceof the first lens L1 and the image plane-side surface of the sixth lensL6La: Distance on an optical axis from the object-side surface of thefirst lens L1 to the image plane IM (an air conversion length isemployed for the filter 10)ν1: Abbe's number of the first lens L1ν2: Abbe's number of the second lens L2ν3: Abbe's number of the third lens L3ν6: Abbe's number of the sixth lens L6Hmax: Maximum image height of the image plane IMDep: Diameter of entrance pupil

Here, it is not necessary to satisfy all of the conditional expressions,and it is achievable to obtain an effect corresponding to the respectiveconditional expression when any single one of the conditionalexpressions is individually satisfied.

According to the embodiment, any of the lenses that lenses that composethe imaging lens is made from a plastic material. Since plasticmaterials are light-weighted and inexpensive, so that they have beenused as lens materials for imaging lenses to be mounted in portabledevices such as smartphones in these years. Typically, plastic lensesare formed by pouring molten plastic material into a mold. For thisreason, fluidity of the plastic material is very important to form thematerial into a desired shape.

Therefore, in order to achieve both miniaturization of the imaging lensand satisfactory correction of aberrations, while securing fluidity uponmolding a lens, the fourth lens L4 preferably satisfies the followingconditional expression: According to the embodiment, the imaging lenssatisfied the following conditional expression:

1.5<E4/T4<3

In the above conditional expression,T4: Thickness of a fourth lens L4 on an optical axisE4: Edge thickness of the fourth lens L4 parallel to the optical axis atthe effective diameter of an object-side surface of the fourth lens L4.

According to the embodiment, lens surfaces of the respective lenses, thefirst lens L1 through the sixth lens L6, are formed as aspheric surfacesas necessary. An equation that express those aspheric surfaces is shownbelow:

$\begin{matrix}{Z = {\frac{C \cdot H^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right) \cdot C^{2} \cdot H^{2}}}} + {\sum\left( {{An} \cdot H^{n}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the above equation:Z: Distance in a direction of the optical axisH: Distance from the optical axis in a direction perpendicular to theoptical axisC: Paraxial curvature (=1/r, r: paraxial curvature radius)k: Conic constantAn: The nth aspheric coefficient

Next, Numerical Data Examples of the imaging lens of the embodiment willbe described. In each Numerical Data Example, f represents a focallength of the whole lens system, Fno represents an F-number, and ωrepresents a half angle of view, respectively. In addition, i representsa surface number counted from the object side, r represents a curvatureradius, d represents a distance on the optical axis between lenssurfaces (surface spacing), n represents a refractive index at an eline, and ν represents an Abbe's number at e line, respectively. Here,aspheric surfaces are indicated with surface numbers i affixed with *(asterisk).

Numerical Data Example 1 Basic Lens Data

TABLE 1 f = 2.78 mm Fno = 2.3 ω = 50.0° i r d n ν [mm] ∞ ∞ L1 1  40.0000.297 1.5371 59.7 f1 = −9.335 2* 4.439 0.413 (=D12) ST 3  ∞ 0.000 L2 4*17.983 0.521 1.5371 59.7 f2 = 3.368 5* −1.991 0.076 (=D23) L3 6* 5.8000.541 1.5371 59.7 f3 = 2.247 7* −1.474 0.030 L4 8* 23.274 0.250 1.668921.9 f4 = −3.431 9* 2.080 0.692 (=D45) L5 10*  2.092 0.250 1.5371 59.7f5 = 563.223 11*  2.018 0.237 (=D56) L6 12*  15.030 0.258 1.6689 21.9 f6= −3.584 13*  2.053 0.160 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.468 (IM) ∞ F1= 2.412 mm F2 = −3.768 mm f23 = 1.451 mm E4 = 0.565 mm T4 = 0.250 mm T5= 0.250 mm T6 = 0.258 mm Φ1A = 2.097 mm Φ4B = 2.558 mm Φ5A = 2.821 mmΦ6B = 3.764 mm L16 = 3.680 mm La = 4.331 mm Hmax = 3.325 mm Dep = 1.210mm

TABLE 2 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 2 0.000E+001.415E−01  1.092E−01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 40.000E+00 9.315E−02 −1.915E−01 1.384E−01 −5.644E−01  0.000E+00 0.000E+000.000E+00 5 0.000E+00 2.212E−02 −5.918E−01 1.060E+00 −9.266E−01 0.000E+00 0.000E+00 0.000E+00 6 0.000E+00 −1.991E−01  −6.045E−011.019E+00 −3.397E−01  0.000E+00 0.000E+00 0.000E+00 7 0.000E+00−1.665E−01   2.064E−01 −2.497E−01  2.528E−01 0.000E+00 0.000E+000.000E+00 8 0.000E+00 1.096E−01  7.046E−02 −1.532E−01  4.876E−020.000E+00 0.000E+00 0.000E+00 9 0.000E+00 1.787E−01 −1.150E−01−1.177E−01  1.698E−01 −9.465E−02  2.721E−02 −3.363E−03  10 0.000E+00−2.876E−01   1.713E−01 −2.746E−01  3.056E−01 −1.833E−01  5.877E−02−8.078E−03  11 0.000E+00 −5.152E−02  −1.341E−01 1.149E−01 −3.289E−02 −6.995E−03  5.082E−03 −6.725E−04  12 0.000E+00 6.469E−03 −5.598E−026.188E−02 −2.252E−02  −2.867E−03  2.899E−03 −3.941E−04  13 0.000E+00−2.052E−01   7.048E−02 −1.576E−02  1.153E−03 −5.886E−04  2.839E−04−3.696E−05 

The values of the respective conditional expressions are as follows:

D12/f=0.149

D23/f=0.027

D12/D23=5.434

f2/f3=1.499

f23/f=0.522

f4/f=−1.234

D45/f=0.249

D45/L16=0.188

Φ5A/Φ4B=1.103

f6/f=−1.289

D56/f=0.085

T5/T6=0.969

La/H max=1.303

f/Dep=2.298

Accordingly, the imaging lens of Numerical Data Example 1 satisfies theabove-described conditional expressions.

FIG. 2 shows a lateral aberration that corresponds to a half angle ofview ω, which is divided into a tangential direction and a sagittaldirection (The same is true for FIGS. 5, 8, 11, 14, 17, and 20).Furthermore, FIG. 3 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. In the astigmatism diagram, anaberration on a sagittal image surface S and an aberration on atangential image surface T are respectively indicated (The same is truefor FIGS. 6, 9, 12, 15, 18, and 21). As shown in FIGS. 2 and 3,according to the imaging lens of Numerical Data Example 1, theaberrations are satisfactorily corrected.

Numerical Data Example 2 Basic Lens Data

TABLE 3 f = 2.66 mm Fno = 2.2 ω = 55.0° i r d n ν [mm] ∞ ∞ L1 1  65.0000.295 1.5371 59.7 f1 = −7.963 2* 4.007 0.462 (=D12) ST 3  ∞ 0.000 L2 4*10.906 0.564 1.5371 59.7 f2 = 2.993 5* −1.851 0.027 (=D23) L3 6* 6.5040.554 1.5371 59.7 f3 = 2.205 7* −1.405 0.029 L4 8* −16.734 0.250 1.668921.9 f4 = −3.234 9* 2.500 0.604 (=D45) L5 10*  2.163 0.250 1.5371 59.7f5 = 127.633 11*  2.143 0.282 (=D56) L6 12*  19.465 0.250 1.6689 21.9 f6= −3.462 13*  2.059 0.160 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.467 (IM) ∞ F1= 2.305 mm F2 = −3.726 mm f23 = 1.371 mm E4 = 0.600 mm T4 = 0.250 mm T5= 0.250 mm T6 = 0.250 mm Φ1A = 2.230 mm Φ4B = 2.597 mm Φ5A = 2.843 mmΦ6B = 3.762 mm L16 = 3.568 mm La = 4.333 mm Hmax = 3.816 mm Dep = 1.193mm

TABLE 4 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 2 0.000E+00 1.272E−01  1.085E−01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+004 0.000E+00  5.752E−02 −2.163E−01 1.711E−01 −7.241E−01  0.000E+000.000E+00 0.000E+00 5 0.000E+00 −5.608E−02 −5.231E−01 1.099E+00−9.814E−01  0.000E+00 0.000E+00 0.000E+00 6 0.000E+00 −2.803E−01−5.734E−01 1.114E+00 −4.050E−01  0.000E+00 0.000E+00 0.000E+00 70.000E+00 −1.824E−01  2.425E−01 −2.830E−01  2.515E−01 0.000E+000.000E+00 0.000E+00 8 0.000E+00  1.537E−01  3.826E−02 −1.570E−01 5.545E−02 0.000E+00 0.000E+00 0.000E+00 9 0.000E+00  2.463E−01−1.849E−01 −7.733E−02  1.600E−01 −9.253E−02  2.587E−02 −3.029E−03  100.000E+00 −2.396E−01  1.424E−01 −2.598E−01  3.013E−01 −1.833E−01 5.917E−02 −8.148E−03  11 0.000E+00 −1.710E−02 −1.611E−01 1.230E−01−3.237E−02  −7.528E−03  5.101E−03 −6.493E−04  12 0.000E+00 −2.771E−03−5.158E−02 6.045E−02 −2.197E−02  −2.753E−03  2.858E−03 −3.947E−04  130.000E+00 −2.035E−01  6.654E−02 −1.453E−02  1.114E−03 −5.971E−04 2.974E−04 −3.965E−05 

The values of the respective conditional expressions are as follows:

D12/f=0.174

D23/f=0.010

D12/D23=17.1

f2/f3=1.357

f23/f=0.515

f4/f=−1.216

D45/f=0.227

D45/L16=0.169

Φ5A/Φ4B=1.095

f6/f=−1.302

D56/f=0.106

T5/16=1.000

La/H max=1.135

f/Dep=2.230

Accordingly, the imaging lens of Numerical Data Example 2 satisfies theabove-described conditional expressions.

FIG. 5 shows a lateral aberration that corresponds to a half angle ofview ω, and FIG. 6 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. As shown in FIGS. 5 and 6, accordingto the imaging lens of Numerical Data Example 2, the aberrations aresatisfactorily corrected.

Numerical Data Example 3 Basic Lens Data

TABLE 5 f = 2.53 mm Fno = 2.1 ω = 60.0° i r d n ν [mm] ∞ ∞ L1 1  20.0000.282 1.5371 59.7 f1 = −6.709 2* 3.038 0.483 (=D12) ST 3  ∞ 0.000 L2 4*11.779 0.598 1.5371 59.7 f2 = 2.536 5* −1.513 0.023 (=D23) L3 6* 19.0430.589 1.5371 59.7 f3 = 2.390 7* −1.361 0.029 L4 8* −12.105 0.249 1.668921.9 f4 = −3.358 9* 2.781 0.564 (=D45) L5 10*  1.798 0.250 1.5371 59.7f5 = 41.223 11*  1.862 0.274 (=D56) L6 12*  33.410 0.249 1.6689 21.9 f6= −3.260 13*  2.041 0.160 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.458 (IM) ∞ F1= 2.226 mm F2 = −3.783 mm f23 = 1.348 mm E4 = 0.614 mm T4 = 0.249 mm T5= 0.250 mm T6 = 0.249 mm Φ1A = 2.335 mm Φ4B = 2.602 mm Φ5A = 2.875 mmΦ6B = 3.767 mm L16 = 3.589 mm La = 4.345 mm Hmax = 4.399 mm Dep = 1.181mm

TABLE 6 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 2 0.000E+00 1.083E−01  1.363E−01 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+004 0.000E+00 −1.696E−02 −2.046E−01 1.107E−01 −8.524E−01  0.000E+000.000E+00 0.000E+00 5 0.000E+00 −1.491E−02 −6.050E−01 1.153E+00−9.687E−01  0.000E+00 0.000E+00 0.000E+00 6 0.000E+00 −1.976E−01−6.009E−01 1.007E+00 −3.531E−01  0.000E+00 0.000E+00 0.000E+00 70.000E+00 −1.825E−01  2.348E−01 −2.527E−01  1.751E−01 0.000E+000.000E+00 0.000E+00 8 0.000E+00  9.946E−02  6.890E−02 −1.368E−01 4.217E−02 0.000E+00 0.000E+00 0.000E+00 9 0.000E+00  1.967E−01−1.028E−01 −1.190E−01  1.691E−01 −9.477E−02  2.709E−02 −3.243E−03  100.000E+00 −2.968E−01  1.787E−01 −2.732E−01  3.046E−01 −1.844E−01 5.846E−02 −7.777E−03  11 0.000E+00 −6 406E−02 −1.332E−01 1.154E−01 −3298E−02  −7.078E−03  5.075E−03 −6 568E−04  12 0.000E+00  1.336E−02−5.541 E−02  6.187E−02 −2.251E−02  −2.856E−03  2.901E−03 −3.949E−04  130.000E+00 −2.049E−01  7.063E−02 −1.563E−02  1.195E−03 −5.811E−04 2.831E−04 −3.848E−05 

The values of the respective conditional expressions are as follows:

D12/f=0.191

D23/f=0.009

D12/D23=21.0

f2/f3=1.061

f23/f=0.533

f4/f=−1.327

D45/f=0.223

D45/L16=0.157

Φ5A/Φ4B=1.105

f6/f=−1.289

D56/f=0.108

T5/T6=1.004

La/H max=0.988

f/Dep=2.142

Accordingly, the imaging lens of Numerical Data Example 3 satisfies theabove-described conditional expressions.

FIG. 8 shows a lateral aberration that corresponds to a half angle ofview ω, and FIG. 9 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. As shown in FIGS. 8 and 9, accordingto the imaging lens of Numerical Data Example 3, the aberrations aresatisfactorily corrected.

Numerical Data Example 4 Basic Lens Data

TABLE 7 f = 2.29 mm Fno = 2.5 ω = 65.0° i r d n ν [mm] ∞ ∞ L1 1  25.0000.297 1.5371 59.7 f1 = −2.829 2* 1.427 0.823 (=D12) ST 3  ∞ 0.000 L2 4*2.525 0.284 1.5371 59.7 f2 = 4.036 5* −14.727 0.028 (=D23) L3 6* 1.9890.719 1.5371 59.7 f3 = 1.866 7* −1.765 0.030 L4 8* −4.416 0.234 1.668921.9 f4 = −4.559 9* 10.068 0.475 (=D45) L5 10*  2.576 0.733 1.5371 59.7f5 = 16.345 11*  3.283 0.473 (=D56) L6 12*  −6.086 0.246 1.6689 21.9 f6= −3.649 13*  4.142 0.160 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.295 (IM) ∞ F1= 2.337 mm F2 = −5.675 mm f23 = 1.382 mm E4 = 0.531 mm T4 = 0.234 mm T5= 0.733 mm T6 = 0.246 mm Φ1A = 2.771 mm Φ4B = 1.901 mm Φ5A = 2.546 mmΦ6B = 3.723 mm L16 = 4.342 mm La = 4.936 mm Hmax = 4.928 mm Dep = 0.933mm

TABLE 8 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 2 0.000E+00 8.079E−02 8.943E−02 0.000E+00  0.000E+00 0.000E+00 0.000E+00 0.000E+004 0.000E+00  5.733E−02 −3.539E−01  5.452E−01 −1.102E+00 0.000E+000.000E+00 0.000E+00 5 0.000E+00 −2.481E−03 −6.686E−01  1.394E+00−1.425E+00 0.000E+00 0.000E+00 0.000E+00 6 0.000E+00 −9.953E−02−3.865E−01  5.781E−01 −2.218E−01 0.000E+00 0.000E+00 0.000E+00 70.000E+00 −2.564E−01 1.857E−01 −5.165E−02  −1.136E−03 0.000E+000.000E+00 0.000E+00 8 0.000E+00  1.748E−02 1.250E−01 −9.787E−02 −3.044E−02 0.000E+00 0.000E+00 0.000E+00 9 0.000E+00  2.157E−011.537E−01 −1.432E−01  −7.412E−02 −1.109E−01  3.267E−01 −4.616E−01  10 0000E+00 −2 133E−01 2.118E−01 −2 878E−01   3.013E−01 −1.850E−01 5.764E−02 −6.998E−03  11 0.000E+00 −8.199E−02 −1.587E−02  1.172E−02−9.381E−03 1.016E−03 7.069E−04 −2.936E−04  12 0.000E+00 −4.251E−02−1.777E−02  8.796E−04  6.072E−03 −6.467E−03  1.065E−04 5.410E−04 130.000E+00 −1.106E−01 2.847E−02 −9.564E−03   3.138E−03 −8.436E−04 1.353E−04 −8.052E−06 

The values of the respective conditional expressions are as follows:

D12/f=0.359

D23/f=0.012

D12/D23=29.4

f2/f3=2.163

f23/f=0.603

f4/f=−1.991

D45/f=0.207

D45/L16=0.109

Φ5A/Φ4B=1.339

f6/f=−1.593

D56/f=0.207

T5/16=2.980

La/H max=1.002

f/Dep=2.454

Accordingly, the imaging lens of Numerical Data Example 4 satisfies theabove-described conditional expressions.

FIG. 11 shows a lateral aberration that corresponds to a half angle ofview ω, and FIG. 12 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. As shown in FIGS. 11 and 12,according to the imaging lens of Numerical Data Example 4, theaberrations are satisfactorily corrected.

Numerical Data Example 5 Basic Lens Data

TABLE 9 f = 2.49 mm Fno = 2.1 ω = 55.0° i r d n ν [mm] ∞ ∞ L1 1  65.0000.285 1.5371 59.7 f1 = −6. 561 2* 3.338 0.522 (=D12) ST 3  ∞ 0.000 L2 4*7.314 0.468 1.5371 59.7 f2 = 2.791 5* −1.843 0.087 (=D23) L3 6* 11.4430.564 1.5371 59.7 f3 = 2.115 7* −1.239 0.029 L4 8* −248.289 0.249 1.668921.9 f4 = −3.078 9* 2.077 0.538 (=D45) L5 10*  2.482 0.341 1.5371 59.7f5 = −12.963 11*  1.742 0.166 (=D56) L6 12*  3.579 0.325 1.6689 21.9 f6= −7.503 13*  2.013 0.160 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.469 (IM) ∞ F1= 2.250 mm F2 = −4.745 mm f23 = 1.336 mm E4 = 0.582 mm T4 = 0.249 mm T5= 0.341 mm T6 = 0.325 mm Φ1A = 2.276 mm Φ4B = 2.460 mm Φ5A = 2.668 mmΦ6B = 3.669 mm L16 = 3.575 mm La = 4.343 mm Hmax = 3.564 mm Dep = 1.169mm

TABLE 10 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 20.000E+00  1.136E−01  1.077E−01  0.000E+00 0.000E+00 0.000E+00 0.000E+000.000E+00 4 0.000E+00  2.267E−02 −1.885E−01 −1.118E−02 −5.695E−01 0.000E+00 0.000E+00 0.000E+00 5 0.000E+00 −3.544E−03 −6.393E−01  1250E+00 −1.171E+00  0.000E+00 0.000E+00 0.000E+00 6 0.000E+00 −2.786E−01−5.026E−01  1.052E+00 −4.037E−01  0.000E+00 0.000E+00 0.000E+00 70.000E+00 −1.208E−01  2.236E−01 −2.748E−01 2.297E−01 0.000E+00 0.000E+000.000E+00 8 0.000E+00  1.646E−01  1.476E−02 −1.637E−01 6.415E−020.000E+00 0.000E+00 0.000E+00 9 0.000E+00  2.133E−01 −1.791E−01−7.378E−02 1.547E−01 −9.141E−02  2.731E−02 −3.602E−03  10 0.000E+00−2.197E−01  1.323E−01 −2.406E−01 3.001E−01 −1.862E−01  5.886E−02−7.962E−03  11 0.000E+00 −7.109E−02 −1.297E−01  1.145E−01 −3.423E−02 −7.412E−03  5.250E−03 −6.378E−04  12 0.000E+00 −2.340E−02 −5.176E−02 6.059E−02 −2.193E−02  −2.752E−03  2.855E−03 −3.937E−04  13 0.000E+00−1.954E−01  6.462E−02 −1.396E−02 1.250E−03 −5.840E−04  2.932E−04−4.443E−05 

The values of the respective conditional expressions are as follows:

D12/f=0.210

D23/f=0.035

D12/D23=6.0

f2/f3=1.320

f23/f=0.537

f4/f=−1.236

D45/f=0.216

D45/L16=0.150

Φ5A/Φ4B=1.085

f6/f=−3.013

D56/f=0.067

T5/T6=1.049

La/H max=1.219

f/Dep=2.130

Accordingly, the imaging lens of Numerical Data Example 5 satisfies theabove-described conditional expressions.

FIG. 14 shows a lateral aberration that corresponds to a half angle ofview ω, and FIG. 15 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. As shown in FIGS. 14 and 15,according to the imaging lens of Numerical Data Example 5, theaberrations are satisfactorily corrected.

Numerical Data Example 6 Basic Lens Data

TABLE 11 f = 2.55 mm Fno = 2.2 ω = 60.0° i r d n ν [mm] ∞ ∞ L1 1  20.0000.277 1.5371 59.7 f1 = −6.734 2* 3.048 0.470 (=D12) ST 3  ∞ 0.000 L2 4*7.066 0.542 1.5371 59.7 f2 = 2.860 5* −1.910 0.113 (=D23) L3 6* −21.2730.557 1.5371 59.7 f3 = 3.005 7* −1.514 0.028 L4 8* 5.988 0.249 1.668921.9 f4 = −8.141 9* 2.805 0.563 (=D45) L5 10*  2.062 0.249 1.5371 59.7f5 = −348.606 11*  1.953 0.302 (=D56) L6 12*  36.355 0.249 1.6689 21.9f6 = −3.243 13*  2.041 0.160 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.452 (IM) ∞F1 = 2.163 mm F2 = −3.348 mm f23 = 1.625 mm E4 = 0.533 mm T4 = 0.249 mmT5 = 0.249 mm T6 = 0.249 mm Φ1A = 2.326 mm Φ4B = 2.702 mm Φ5A = 2.903 mmΦ6B = 3.839 mm L16 = 3.599 mm La = 4.349 mm Hmax = 4.433 mm Dep = 1.183mm

TABLE 12 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 20.000E+00  1.060E−01  1.034E−01 0.000E+00 0.000E+00 0 000E+00 0 000E+000.000E+00 4 0.000E+00 −2.672E−02 −2.048E−01 6.523E−02 −6.922E−01 0.000E+00 0.000E+00 0.000E+00 5 0.000E+00 −4.406E−02 −6.568E−011.188E+00 −1.012E+00  0.000E+00 0.000E+00 0.000E+00 6 0.000E+00−1.898E−01 −5.761E−01 1.017E+00 −3.654E−01  0 000E+00 0 000E+000.000E+00 7 0.000E+00 −2.372E−01  2.461E−01 −2.351E−01  1.655E−010.000E+00 0.000E+00 0.000E+00 8 0.000E+00  6.785E−02  5.892E−02−1.329E−01  4.258E−02 0.000E+00 0.000E+00 0.000E+00 9 0.000E+00 2.130E−01 −1.065E−01 −1.240E−01  1.684E−01 −9.393E−02  2.759E−02−3.419E−03  10 0.000E+00 −2.697E−01  1.788E−01 −2.730E−01  3.048E−01−1.842E−01  5.848E−02 −7.848E−03  11 0.000E+00 −5.522E−02 −1.344E−011.143E−01 −3.301E−02  −6.984E−03  5.112E−03 −6.594E−04  12 0.000E+00 3.781E−03 −5.498E−02 6.225E−02 −2.251E−02  −2.898E−03  2.889E−03−3.905E−04  13 0.000E+00 −2.044E−01  6.994E−02 −1.564E−02  1.283E−03−5.497E−04  2.844E−04 −4.317E−05 

The values of the respective conditional expressions are as follows:

D12/f=0.184

D23/f=0.044

D12/D23=4.159

f2/f3=0.952

f23/f=0.637

f4/f=−3.193

D45/f=0.221

D45/L16=0.156

Φ5A/Φ4B=1.074

f6/f=−1.272

D56/f=0.118

T5/16=1.000

La/H max=0.981

f/Dep=2.156

Accordingly, the imaging lens of Numerical Data Example 6 satisfies theabove-described conditional expressions.

FIG. 17 shows a lateral aberration that corresponds to a half angle ofview ω, and FIG. 18 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. As shown in FIGS. 17 and 18,according to the imaging lens of Numerical Data Example 6, theaberrations are satisfactorily corrected.

Numerical Data Example 7 Basic Lens Data

TABLE 13 f = 2.36 mm Fno = 2.1 ω = 60.0° i r d n ν [mm] ∞ ∞ L1 1* 1.4080.299 1.5371 59.7 f1 = −3.286 2* 0.725 0.654 (=D12) ST 3  ∞ 0.000 L2 4*9.059 0.610 1.5371 59.7 f2 = 2.212 5* −1.335 0.028 (=D23) L3 6* 2.0400.663 1.5371 59.7 f3 = 1.962 7* −1.931 0.038 L4 8* −1.282 0.249 1.668921.9 f4 = −2.890 9* −4.101 0.744 (=D45) L5 10*  2.682 0.250 1.5371 59.7f5 = −8.991 11*  1.668 0.116 (=D56) L6 12*  −5.355 0.345 1.6689 21.9 f6= −8.469 13*  −100.249 0.100 14  ∞ 0.210 1.5187 64.0 15  ∞ 0.414 (IM) ∞F1 = 2.012 mm F2 = −4.409 mm f23 = 1.125 mm E4 = 0.603 mm T4 = 0.249 mmT5 = 0.250 mm T6 = 0.345 mm Φ1A = 2.197 mm Φ4B = 2.349 mm Φ5A = 2.629 mmΦ6B = 3.830 mm L16 = 3.995 mm La = 4.648 mm Hmax = 4.108 mm Dep = 1.120mm

TABLE 14 Aspherical surface data i k A4 A6 A8 A10 A12 A14 A16 10.000E+00 −1.308E−01 −4.626E−02  0.000E+00 0.000E+00 0.000E+00 0.000E+000.000E+00 2 0.000E+00 −1.268E−01 4.746E−02 0.000E+00 0.000E+00 0.000E+000.000E+00 0.000E+00 4 0.000E+00 −8.456E−03 1.187E−01 −3 366E−01 3.879E−01 0.000E+00 0.000E+00 0.000E+00 5 0.000E+00 −2.579E−02 6.158E−03−3.372E−01  2.950E−01 0.000E+00 0.000E+00 0.000E+00 6 0.000E+00 3.409E−02 5.121E−03 1.884E−02 −5.584E−02  0.000E+00 0.000E+00 0.000E+007 0.000E+00 −3.420E−02 4.940E−01 −4.674E−01  1.493E−01 0.000E+000.000E+00 0.000E+00 8 0.000E+00  6.914E−01 −8.402E−01  6.402E−01−1.493E−01  0.000E+00 0.000E+00 0.000E+00 9 0.000E+00  7.434E−01−9.196E−01  5.477E−01 4.575E−02 −2.037E−01  7.586E−02 −7.733E−03  100.000E+00 −4.619E−01 3.243E−01 −7.935E−02  1.707E−02 −8.580E−02 6.566E−02 −1.514E−02  11 0.000E+00 −3 811E−01 2.528E−01 −1.274E−01 4.498E−02 −1 447E−02  1.940E−03 4.905E−05 12 0.000E+00  3.455E−01−3.654E−01  1.768E−01 −2.492E−02  −1.415E−02  6.081E−03 −6.664E−04  130.000E+00  1.459E−01 −1.224E−01  3.579E−02 −2.005E−03  −1.361E−03 2.959E−04 −1.786E−05 

The values of the respective conditional expressions are as follows:

D12/f=0.277

D23/f=0.012

D12/D23=23.4

f2/f3=1.127

f23/f=0.477

f4/f=−1.225

D45/f=0.315

D45/L16=0.186

Φ5A/Φ4B=1.119

f6/f=−3.589

D56/f=0.049

T5/T6=0.725

La/H max=1.131

f/Dep=2.107

Accordingly, the imaging lens of Numerical Data Example 7 satisfies theabove-described conditional expressions.

FIG. 20 shows a lateral aberration that corresponds to a half angle ofview ω, and FIG. 21 shows a spherical aberration (mm), astigmatism (mm),and a distortion (%), respectively. As shown in FIGS. 20 and 21,according to the imaging lens of Numerical Data Example 7, theaberrations are satisfactorily corrected.

According to the embodiment of the present invention, the imaging lenseshave very wide angles of view (2ω) of 100° or greater. According to theimaging lens of the embodiment, it is possible to take an image over awider range than that taken by a conventional imaging lens, while havinga small size.

According to the imaging lenses of the embodiment, the Fnos are as smallas 2.1 to 2.5. According to the imaging lens of the embodiment, it isachievable to obtain a sufficiently bright image without providing theabove-described electrical circuit to reduce noises in the imagingelement.

Accordingly, when the imaging lens of the embodiment is mounted in animaging optical system, such as onboard cameras, smartphones, digitalcameras, digital video cameras, network cameras, TV conference cameras,fiberscopes, and capsule endoscopes, it is possible to attain both highperformance and downsizing of the cameras.

Accordingly, the present invention is applicable to an imaging lens formounting in a relatively small cameras, such as onboard cameras,smartphones and cellular phones, digital cameras, digital video cameras,network cameras, TV conference cameras, fiberscopes, and capsuleendoscopes, it is possible to attain both high performance anddownsizing of the cameras.

The disclosure of Japanese Patent Application No. 2017-237736, filed onDec. 12, 2017, is incorporated in the application by reference.

While the present invention has been explained with reference to thespecific embodiment of the present invention, the explanation isillustrative and the present invention is limited only by the appendedclaims.

What is claimed is:
 1. An imaging lens comprising: a first lens havingnegative refractive power; an aperture stop; a second lens; a thirdlens; a fourth lens having negative refractive power; a fifth lens; anda sixth lens, arranged in this order from an object side to an imageplane side, wherein said fifth lens is formed in a shape so that asurface thereof on the object side is convex at a paraxial regionthereof, said sixth lens is formed in a meniscus shape at a paraxialregion thereof, and said first lens has an Abbe's number ν1 and saidsixth lens has an Abbe's number ν6 so that the following conditionalexpressions are satisfied:40<ν1<75,10<ν6<40.
 2. The imaging lens according to claim 1, wherein said firstlens is disposed away from the second lens by a distance D12 on anoptical axis thereof so that the following conditional expression issatisfied:0.05<D12/f<0.8, where f is a focal length of a whole lens system.
 3. Theimaging lens according to claim 1, wherein said second lens is disposedaway from the third lens by a distance D23 on an optical axis thereof sothat the following conditional expression is satisfied:0.001<D23/f<0.3, where f is a focal length of a whole lens system. 4.The imaging lens according to claim 1, wherein said first lens isdisposed away from the second lens by a distance D12 on an optical axisthereof, and said second lens is disposed away from the third lens by adistance D23 on an optical axis thereof so that the followingconditional expression is satisfied:2<D12/D23<30.
 5. The imaging lens according to claim 1, wherein saidsecond lens and said third lens has a composite focal length f23 so thatthe following conditional expression is satisfied:0.2<f23/f<1, where f is a focal length of a whole lens system.
 6. Theimaging lens according to claim 1, wherein said sixth lens has a focallength f6 so that the following conditional expression is satisfied:−5<f6/f<−0.5, where f is a focal length of a whole lens system.
 7. Theimaging lens according to claim 1, wherein said fifth lens has athickness T5 on an optical axis thereof and said sixth lens has athickness T6 on an optical axis thereof so that the followingconditional expression is satisfied:0.5<T5/T6<3.5.
 8. An imaging lens comprising: a first lens havingnegative refractive power; an aperture stop; a second lens; a thirdlens; a fourth lens having negative refractive power; a fifth lens; anda sixth lens, arranged in this order from an object side to an imageplane side, wherein said third lens is formed in a shape so that asurface thereof on the object side is convex at a paraxial regionthereof, said fifth lens is formed in a shape so that a surface thereofon the object side is convex at a paraxial region thereof, said sixthlens is formed in a meniscus shape at a paraxial region thereof, andsaid sixth lens has an Abbe's number ν6 so that the followingconditional expression is satisfied:10<ν6<40.
 9. The imaging lens according to claim 8, wherein said firstlens is disposed away from the second lens by a distance D12 on anoptical axis thereof so that the following conditional expression issatisfied:0.05<D12/f<0.8, where f is a focal length of a whole lens system. 10.The imaging lens according to claim 8, wherein said second lens isdisposed away from the third lens by a distance D23 on an optical axisthereof so that the following conditional expression is satisfied:0.001<D23/f<0.3, where f is a focal length of a whole lens system. 11.The imaging lens according to claim 8, wherein said first lens isdisposed away from the second lens by a distance D12 on an optical axisthereof, and said second lens is disposed away from the third lens by adistance D23 on an optical axis thereof so that the followingconditional expression is satisfied:2<D12/D23<30.
 12. The imaging lens according to claim 8, wherein saidsecond lens and said third lens has a composite focal length f23 so thatthe following conditional expression is satisfied:0.2<f23/f<1, where f is a focal length of a whole lens system.
 13. Theimaging lens according to claim 8, wherein said sixth lens has a focallength f6 so that the following conditional expression is satisfied:−5<f6/f<−0.5, where f is a focal length of a whole lens system.
 14. Theimaging lens according to claim 8, wherein said fifth lens has athickness T5 on an optical axis thereof and said sixth lens has athickness T6 on an optical axis thereof so that the followingconditional expression is satisfied:0.5<T5/T6<3.5.
 15. An imaging lens comprising: a first lens havingnegative refractive power; an aperture stop; a second lens; a thirdlens; a fourth lens; a fifth lens; and a sixth lens, arranged in thisorder from an object side to an image plane side, wherein said thirdlens is formed in a shape so that a surface thereof on the object sideis convex at a paraxial region thereof, said fourth lens is formed in ameniscus shape at a paraxial region thereof, said fifth lens is formedin a shape so that a surface thereof on the object side is convex at aparaxial region thereof, said sixth lens is formed in a meniscus shapeat a paraxial region thereof, and said sixth lens has an Abbe's numberν6 so that the following conditional expression is satisfied:10<ν6<40.
 16. The imaging lens according to claim 15, wherein said firstlens is disposed away from the second lens by a distance D12 on anoptical axis thereof so that the following conditional expression issatisfied:0.05<D12/f<0.8, where f is a focal length of a whole lens system. 17.The imaging lens according to claim 15, wherein said second lens isdisposed away from the third lens by a distance D23 on an optical axisthereof so that the following conditional expression is satisfied:0.001<D23/f<0.3, where f is a focal length of a whole lens system. 18.The imaging lens according to claim 15, wherein said first lens isdisposed away from the second lens by a distance D12 on an optical axisthereof, and said second lens is disposed away from the third lens by adistance D23 on an optical axis thereof so that the followingconditional expression is satisfied:2<D12/D23<30.
 19. The imaging lens according to claim 15, wherein saidsecond lens and said third lens has a composite focal length f23 so thatthe following conditional expression is satisfied:0.2<f23/f<1, where f is a focal length of a whole lens system.
 20. Theimaging lens according to claim 15, wherein said sixth lens has a focallength f6 so that the following conditional expression is satisfied:−5<f6/f<−0.5, where f is a focal length of a whole lens system.